Factor XI (fXI) is the zymogen of a trypsin-like protease (fXIa) that contributes to thrombin generation by catalyzing factor IX (fIX) activation. Despite playing a limited role in hemostasis, a variety of data now make a compelling case that fXI contributes to arterial and venous thrombosis and is, therefore, an attractive candidate for targeted therapy to treat or prevent thromboembolic disease. FXI has a number of structural features that distinguish it from vitamin K-dependent coagulation proteases. The protein is a dimer of two identical subunits, each of which has two sites for binding to platelet receptors, and two anion-binding sites. The functional importance of these features remains to be established, and none are required for normal fXI function in plasma clotting assays such as the activated partial thromboplastin time (aPTT) assay. We propose that the dimeric fXI structure and interactions with platelets, and polyanions are key to regulation of fXI activation and fXIa activiy in flowing blood. In the aPTT assay, fXI is activated by factor XIIa (fXIIa), and fXIa in turn activates fIX. The absence of abnormal bleeding in fXII deficiency indicates other proteases must be capable of activating fXI in vivo. For example, -thrombin can activate fXI. However, there is mounting evidence that fXI activation by fXIIa contributes to thrombosis. FXII deficient and fXI deficient mice are resistant to injury-induced arterial and venous occlusion, and similar observations have been made in baboons treated with anti-fXI or fXII antibodies. We hypothesize that both fXI and fXII contribute to growth of an occlusive thrombus in flowing human blood through thrombin-dependent processes. Furthermore, we propose that an interaction between fXI and the platelet or a platelet product is required to support thrombus growth under flow through generation of fXIa. FXI activation is enhanced by polymers of inorganic phosphate (poly-P) released from platelet dense granules. Poly-P appears to bind to both of the anion binding sites on fXI, potentiating its activation by thrombin, fXII and autoactivation, and may down-regulate fXIa inhibition by serpins. In Aim 1, we will use an ex vivo flow model to study the importance of fXI and fXII to thrombus formation in whole blood. This model will also be used to study the importance of the fXI dimer, fXI platelet binding sites, and fXI anion binding sites to thrombus formation, using a well characterized panel of recombinant fXI molecules. In Aim 2, we will study the importance of the fXI anion binding sites to fXI activity in two models of arterial thrombosis, and one model of venous thromboembolism, in mice; and will assess the possibility that the anion binding sites are involved in establishing non-circulating pool of fXI through interactions with vessel wall glycosaminoglycans. In Aim 3, we will determine the importance of the fXI anion binding sites to heparin-dependent regulation of fXIa by plasma serpins, and evaluate the capacity of poly-P to down-regulate serpin-mediated inhibition. Results from the studies presented in this proposal will elucidate mechanisms by which fXI and fXII contribute to thrombus formation, and may identify novel processes that can be exploited for therapeutic purposes.